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WHY CUSTOM LENGTH BOATS?

We all come in different sizes, shapes and vary in degree of fitness and strength. Most boats and kits are manufactured and plans come for one or two standard sizes. Kayak manufacturers have acceded to big and little people with popular and good boats that have been stretched or shrunk one or more dimensions to suit. We usually just pick the one we feel most comfortable in, or the one whose performance specs appeal to our self-image.

We also have particular uses that require specific lengths or widths to get a particular carrying capacity, or a storage space to fit, or a weight we can lift.

And we all like to dream a little too. We see ourselves ahead of the pack, or multi-day voyaging in remote locations. I read Mind over Water last year and Rowing to Latitude, and On Celtic Tides this year. Taking an armchair journey fires up our imagination.

In between building projects, I did some researching on the web1 looking up "rowing physics" and "kayaking physics", trying to get some additional design data. I did pick up some great sliding seat rowing data to add to an old chart of human output in horsepower, versus time, for bicyclers, rowers and laborers from my college biotechnology text. Then I added that data to a chart by the Trireme Trust on their data for fixed seat rowing, man-powered flight and bicyclers. And then I added some data for Olympic kayakers from an old article. All plotted on log-log paper to help flatten the curves into nearly straight lines of decreasing net horsepower with increasing length of effort.

log chart
Chart thumbnail

Olympic kayakers produce about the same effective horsepower (arms only) as lightweight women Olympic rowers (arms and legs). Scattered data for fixed seat rowers says they produce far less, but a trireme rowed by college students and navy fill-ins, a 31-foot cutter rowed by strapping sailors nearly sixty years ago, or the Etonian 8 of 1870 is not quite in the same league of data. Even so, the fixed seat rower data line is about 55% of the Olympic kayaker line.

In fact, significant improvements in performance have occurred in the last two decades due to advances in training more than boats or rowers. In fact, "eights" are going so fast now that the bottom on Olympic courses is retarding their speed because the waves are constrained by the depth.

The rowing physics data triggered an "Aha!" Since you can predict horsepower output per pound2/3 (equivalent ratio to lung capacity) for heavy-weight men, light-weight men, heavy-weight women, and light-weight women Olympic rowers at the top of their training; and you can then get it for untrained men and women as a percent of Olympians performance (about 60%), then you can match their projected output in horsepower to a boat's total resistance in pounds times it's speed in feet per second divided by 550. So you can theoretically get a boat that "fits" a rower or paddler in both butt size and optimum length and resistance to go their preferred training time or all day, if desired.

resistant plot
Drag curve thumbnail

Since wave-making resistance of boats has maxima and minima as the curve increases with speed as the various wave systems generated by the hull overlap and then partially cancel each other, you place a minima at their desired horsepower for your time or distance. Voila!

Calculating drag and horsepower is not too difficult with the right books, data tables and hand held calculator. There are a couple of fairly accurate ways for small craft, but the slickest at present is an Excel spread sheet developed by Matt Brose of Mariner kayaks, and used to compare kayak resistances in SeaKayaker magazine. It works for other boats as well. It can be accessed on line2 and used, or downloaded and used on a pc or mac.

drag calc
Drag Calc thumbnail

(At some point, I must recognize that many of us are math challenged, or don't just dive into complex Excel spread sheets like we know what we are doing. Find someone you know who does! Engineers and scientists love a challenge.)

Finding your horsepower output is a little trickier. Collegiate teams do lactic acid build-up testing on an ergometer rowing machine. They row timed sets with increasing resistances, heart rate monitoring and blood testing to find where they top out in ability. Our muscles burn oxygen and one waste product is lactic acid. When it builds up, we feel that burning sensation in our arms or legs that inhibits further performance. We can move the level through proper exercises and distance training. They check blood chemistry. We non-collegiate, non-rich can measure our heart rates…a little less accurate.

One estimate for maximum heartbeat is 220 minus your age, plus or minus 13. For me, that's about 160, between 147 and 173. Since I have a resting heartbeat (62) below average of 72, I would expect to be closer to 150.

In short bursts, we create a short-term oxygen debt (anaerobic), which we make up by breathing hard afterwards. In long distances, rowing, cycling, hiking, etc, we reach some oxygen equilibrium (aerobic) with our heart, lungs and muscles. Some people have different amounts of fast twitch, and slow-twitch muscle tissue pre-disposing them for good distance running or rowing, while others are good in the sprints. Similar to the very few who can summit Mt. Everest without oxygen, a genetic predisposition to success in certain sports.

We all have some maximal heart rate, at which point increased effort does not raise our heartbeat, blood flows no faster, and oxygen gets to our muscles at the same rate…we top out aerobically. This too can be improved with exercise.

As we age, our lung capacity and inhalation and exhalation rates decrease. One estimate is that we lose 30% of our lung capacity between age 30 and 70, or 8 to 10% per decade for the average Joe. Another estimate used in hospitals is 5 ml per year on the spirometer chart for all people, independent of size or gender! Another is virtually no decline for well-seasoned people who work out regularly.

Determining heart rate is should be sport specific: You can determine heart rate by running stairs, but it's a good number for stairs. You can row on an ergometer. I paddle or row a known distance (1/2 mile, 1 mile, 2 miles) at a fixed rate and take my pulse and time to cover the distance.

My "target heart rate" is between 120 and 130 for training purposes at the lowest level of fitness according to some rowing training books. That is found by taking between 60 and 70 percent of my maximum heart rate minus my resting heart rate, and adding back my resting heart rate. ((150-60) * 0.7) + 60 = 123.

At age sixty, my theoretical maximum heart rate is about 150. A one-mile row at about 85% yields a pulse of 128. Since I know the resistance of my boats, I can calculate pounds of drag and feet per second, and thus horsepower vs. heart rate for aerobic performance.

You can do this too. You just need a watch, a map giving some fixed distances for rowing and a table of resistances for your boat. If you are a kayaker, you can look it up on line, or estimate it from very similar boats, or use the calculator on line. If you are a rower, you might have to work a little harder at getting resistance data for your boat from the manufacturer etc. Or you can have Long Lake Wooden Boats do the calculations for you. You can also order stock plans and customize the lengths to some extent.

Why bother? So you can finally get a boat that really fits you, or one that you can train into for fitness or distance rowing or paddling. Or one you can get onto your roof-rack.

1www.atm.ox.ac.uk/rowing/physics, www.atkinsopht.com/row, http://filebox.vt.edu/eng/mech/tidwel/me4016/final/dynamic 2www.marinerkayaks.com/…
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